ee535

7/28/2019 EE535

http://slidepdf.com/reader/full/ee535 1/33

Manikandan.P

CUFE–

EEE Department

7/28/2019 EE535


Single Line Diagram

7/28/2019 EE535


7/28/2019 EE535


Duplicate Lines:

The transmission is generally along with additional lines in parallel. These linesare called duplicate lines.

Factors - Design of transmission lines

Voltage Levels

Resistance

Reactance

Line PerformanceInterference

withneighboring

circuits

Strength of thesupports

Sag

7/28/2019 EE535


High voltages of the order of66 kV

132 kV220 kV400 kVare used for transmitting power by 3 phase 3 wire overhead system. This issupplied to substations usually at the out skirts of major distribution center orcity

Primary Transmission

Secondary transmission

The primary voltage is reduced to low values of the order of

3.3 kV11 kV33 kVfor secondary transmission

7/28/2019 EE535


Primary Distribution

The transmission lines or inner connectors terminate at large main substationsfrom which the power is distributed to small secondary substations scatteredthroughout the load area. The voltage may range from 11 kV to 132 kV

Secondary distribution:

Low-voltage network laid along the streets, localities and over the rural areas.

From these sources connections to individual customers are provided

The circuit used for this purpose is 3 phase 4 wire, 440 V/220 V from which either3 phase 440 V or single phase 220 V supply to the consumers may be provided

7/28/2019 EE535


Tasks of Transmission systems:

Transmission of electric field or Power at specified Voltage and Frequency

Control of Flow of Power with respect to magnitude and direction

Ensuring steady state stability and transient stability of the transmission line andassociated AC Networks

Control and flow of Reactive Power

Voltage control at Sending end and Receiving end of Transmission lines

Assistance in frequency control

Security of Supply by Feeding at Various points providing adequate line capacityfacility for alternative transmission

Data transmission through Power line carrier communication channel

Minimize Transmission losses by selecting shorter transmission paths

7/28/2019 EE535


Choice of transmission systems:

7/28/2019 EE535


Capital Cost of Line Conductors ,Towers ,Insulators and Installation

Substation Costs – Transformers,Switchgears, Substation area Buildings

Cost of Energy Losses and Maintenance

ECONOMICAL CONSIDERATIONS

7/28/2019 EE535


Length of the Transmission Lines and total Power to beTransferred

Control Power Magnitude and rate of change

Existing Networks and Long Term plans

Stability considerations related with power flowand frequency disturbances

Type of Lines : OHL,UGCL,SCL

TECHNICAL CONSIDERATIONS

7/28/2019 EE535


AC and DC

http://d/TD/makz/HVDC___Connecting_remote_power_generation_to_the_market.mp4

http://d/TD/makz/HVDC___Connecting_remote_power_generation_to_the_market.mp4

7/28/2019 EE535


Possible to build up High A.C voltage levels,Using high speed a.c generators

A.C voltages can be raised or lowered byTransformers

Motors running on A.C are simple inconstruction,cheaper and require less attentionfrom mainteinance point of view

7/28/2019 EE535


Construction of Transmission line is complicated

Resistance of AC line is higher due to the skin effect causing more voltage drop

Inductance also causes a drop

Copper requirement

Problem of synchronization of alternators

Problem due to charging current

7/28/2019 EE535


Suitable for long distance transmission

No Stability Problems

Line length is not limitation as there is no charging current in theDC transmission

Better voltage regulation, Less Corona Loss and Line Loss

Shunt compensation is not required

Fault current is reduced by the Converters

7/28/2019 EE535


Length is less than 500 km, Power transmission is noteconomical

Circuit Breaking is difficult and expensive

Considerable Reactive Power is required by converterStations

Harmonics are generated and so filters are mandatory

Maintenance of Insulators of HVDC system is more

7/28/2019 EE535


Traditionally, AC lines have no provision for the control of power Flow

Fortunately, ac lines have inherent power flow control as the power Flow isdetermined by the power at the sending end or receiving end.

X is the series line reactance.The power injected by the power station determinesthe flow of power in the line.

7/28/2019 EE535


This results in the creation of a mesh in the transmissionnetwork. This improves the system reliability, as tripping of any oneline does not result in curtailment of the load.

In general, it can be stated that in an uncontrolled AC transmission networkwith loops (to improve system reliability), the power flows in individual linesare determined by KVL and do not follow the requirements of the contracts(between energy producersand customers).

7/28/2019 EE535


Control the power Flow in a AC transmission line to(a)enhance power transfer capacity(b) to change power Flow under dynamic conditions (subjected to disturbancessuch as sudden increase in load, line trip or generator outage) to ensure system

stability and security.

As the line length increases, X increases in a linear fashion and P max reduces

V1 and V2 denote the voltages at either end of the interconnection, whereas Deltamax denotes the angular difference of the said voltages. X isthe reactance of the transmission circuit,

7/28/2019 EE535


The series compensation using series connected capacitors increasesPmax as the compensated value of the series reactance (Xc) is given by

where kse is the degree of series compensation. The maximum value of ksethat can be used depends on several factors including the resistance of theconductors. Typically kse does not exceed 0.7.

7/28/2019 EE535


faults caused by lightning discharges

variations caused by the weather (ambient

temperature)

sudden increase/decrease in the power flow

swinging of generator rotors

transient instability

7/28/2019 EE535


The FACTS controllers can be classified as

1. Shunt connected controllers

2. Series connected controllers 3. Combined series-series controllers

4. Combined shunt-series controllers

7/28/2019 EE535


Depending on the power electronic devices used in the control, theFACTS controllers can be classified as

(A) Variable impedance type(B) Voltage Source Converter (VSC) .

Variable impedance type

(i) Static Var Compensator (SVC), (shunt connected)(ii) Thyristor Controlled Series Capacitor or compensator(TCSC), (seriesconnected)(iii) Thyristor Controlled Phase Shifting Transformer (TCPST)of StaticPST (combined shunt and series)

(i) Static synchronous Compensator (STATCOM)(shunt connected)(ii) Static Synchronous Series Compensator (SSSC)(series connected)(iii) Interline Power Flow Controller (IPFC)(combined series-series)

(iv) Unified Power Flow Controller (UPFC)(combined shunt-series)

Voltage Source Converter(VSC)

7/28/2019 EE535


Some of the special purpose FACTS controllers are

(a) Thyristor Controller Braking Resistor (TCBR)(b) Thyristor Controlled Voltage Limiter (TCVL)(c) Thyristor Controlled Voltage Regulator (TCVR)(d) Interphase Power Controller (IPC)

An electrical resonant frequency on an alternating-current transmission line that is

less than the line frequency, and results from the insertion of series capacitors tocancel out part of the line and system reactance. - Sub synchronous Resonance

Resonance of a circuit involving capacitors and inductors occurs because thecollapsing magnetic field of the inductor generates an electric current in itswindings that charges the capacitor, and then the discharging capacitor provides

an electric current that builds the magnetic field in the inductor

7/28/2019 EE535


By controlled series compensation, we imply dynamic control of the degreeof series compensation in a long line. This can be achieved in two ways as

1. Discrete control using TSSC (Thyristor Switched Series Capacitor)

2. Continuous control using(a) TCSC or(b) GTO Thyristor Controlled Series Capacitor (GCSC)

7/28/2019 EE535


TCSCThyristor Controlled Series Compensation

7/28/2019 EE535


The impact of TCSC in a power transmission grid canbe summarized as

Balancing of load flows

This enables the load flow on parallel circuits and different voltage levels to beoptimized, with a minimum of power wheeling, the best possible utilization ofthe lines, and a minimizing of overall system losses at the same time.

Increasing power oscillation damping, and voltage stability

This enables a maximizing of system availability as well as of powertransmission capability over existing as well as new lines.Thus, more power can be transmitted over less lines, with a saving of money as

well as of environmental impact of the transmission link.

Mitigation of sub synchronous resonance risk

Sub synchronous resonance (SSR) is a phenomenon which can be associatedwith series compensation under certain adverse conditions

7/28/2019 EE535


by introducing a capacitive reactance in the denominator of P max it is possible toachieve a decrease of the angular separation with power transmission capabilityunaffected, i.e. an increase of the angular stability of the link.

Closer analysis reveals, however, that the reactive power contribution from acapacitive element in series with the line acts to improve the reactive powerbalance of the circuit, and thereby to bring about a stabilization of thetransmission voltage.

http://d/TD/makz/Best_Explanation_for_KVAr__with_an_Example__Must_Watch__Part_2.mp4

7/28/2019 EE535


With the reactance of the capacitive element,i.e. the series capacitor equal to XC and the inductive reactance of the line equalto XL, we can define the degree of series compensation,

k = XC / XL

7/28/2019 EE535


Impedance of TCSC

current through theTCR

Since the losses are neglected, the impedance of TCSC is purely reactive. The

capacitive reactance of TCSC is obtained from

7/28/2019 EE535


Operation of TCSC

Bypass mode :

Thyristor valves are gated for 180 degree conduction (in each direction)

The net reactance of the module is slightly inductive as the susceptance of the reactoris larger than that of the capacitor.

This mode is used mainly for protecting the capacitor against overvoltages (during

transient overcurrents in the line).This mode is also termed as TSR (ThyristorSwitched Reactor) mode.

7/28/2019 EE535


Inserted with Vernier Control

In this operating mode, the thyristor valves are gated in the region of (alpha min < 0 < 90 degree) such that they conduct for the part of a cycle.

The effective value of TCSC reactance (in the capacitive region) increasesas the conduction angle increases from zero. Alpha min is above the valueof alpha corresponding to the parallel resonance of TCR and the capacitor(at fundamental frequency).

In the inductive vernier mode, the TCSC (inductive) reactance increases asthe conduction angle reduced from 180 deg.

7/28/2019 EE535


A TCSC is placed on a 500kV, long transmission line, to improve powertransfer. Without the TCSC the power transfer is around 110MW The nominalcompensation is 75%, i.e. assuming only the capacitors (firing angle of 90deg).

The TCSC can operate in capacitive or inductive mode, although the latter is

rarely used in practice. Since the resonance for this TCSC is around 58degfiring angle, the operation is prohibited in firing angle range 49deg - 69deg.

Note that the resonance for the overall system (when the line impedance isincluded) is around 67deg. The capacitive mode is achieved with firing angles69-90deg. The impedance is lowest at 90deg, and therefore power transfer

increases as the firing angle is reduced.

In capacitive mode the range for impedance values is approximately 120-136Ohm. This range corresponds to approximately 490-830MW power transferrange (100%-110% compensation). Comparing with the power transfer of 110MW with an uncompensated line, TCSC enables significant improvement in

power transfer level

7/28/2019 EE535


For the first 0.5s, the TCSC is bypassed (assuming a circuit breaker), and thepower transfer is 110 MW.

At 0.5s TCSC begins to regulate the impedance to 128 Ohm and this increasespower transfer to 610MW

At 2.5s a 5% change in the reference impedance is applied. The responseindicates that TCSC enables tracking of the reference impedance and thesettling time is around 500ms

At 3.3s a 4% reduction in the source voltage is applied,

It is seen that the TCSC controller compensates for these disturbances and the TCSC

impedance stays constant The TCSC response time is 200ms 300ms

ee535

Documents